MTBE Drinking Water Standards

a primary MCL of 13 micrograms per liter (µg/L), established in 2000, that addresses health concerns, as explained in the final statement of reasons. Its 1999 public health goal is 13 µg/L, corresponding to the de minimis cancer risk derived from animal studies

a secondary MCL of 5 µg/L, established in 1999. The secondary MCL addresses taste and odor concerns.

a detection limit for purposes of reporting (DLR) of 3 µg/L. The DLR is the level at which the Division of Drinking Water is confident about the quantity being reported. Results at or above the DLR are required to be reported to CDPH; some laboratories may report results at lower concentrations.

Monitoring Results

MTBE was found in some drinking water sources in the early and mid-1990s, including two wells under development but not put into use that had peak concentrations of 23 and 500 µg/L at the Presidio of San Francisco in 1990, and in four wells in Santa Monica with peak concentrations of 87 to 610 µg/L in 1995-1996

Thereafter, CDHS (the California Department of Health Services, then the California Department of Public Health, CDPH, and now the Division of Drinking Water (DDW), SWRCB) adopted a 1997 regulation that included MTBE as an unregulated chemical for which monitoring was required to be done by certain public water systems. Subsequently, required monitoring has been associated with compliance with MTBE's MCLs..

The DDW water quality database includes MTBE analytical results reported over 15,000 sources, where "sources" may include both raw and treated drinking water wells and surface water sources, distribution systems, blending reservoirs, and other sampled entities. Active, standby, inactive, and abandoned or destroyed sources are included. Nearly all of the results are non-detects

This update presents MTBE findings since January 1, 2000, and shows that 69 sources have reported MTBE detections (PDF) greater than the DLR. The sources are in 27 counties, as shown in Table 1, which also shows 24 sources with a peak detection greater than MTBE's primary MCL, and 31 sources with a peak level greater than the secondary MCL but lower than the primary MCL. Readers should be advised that public water systems may not serve drinking water that exceeds the primary MCL. In addition, some of these sources are inactive, abandoned or destroyed.

ince 2000, the highest findings have been 400 µg/L in Monterey County and 112 µg/L in San Diego County. Both occurred in 2004.

These findings are helpful in identifying areas in which MTBE has affected sources of drinking water, but they should not be interpreted as representative of water being served by public water systems. Consumers interested in finding out more about the quality of their drinking water should refer to their water systems' annual consumer confidence reports (CCRs). Many CCRs for California water systems are available on the US EPA's website.

Table 1. Peak concentrations of MTBE reported in sources of drinking water (2000-2009)
* These data include results from active, standby, inactive and abandoned or destroyed sources.

County

>13 µg/L

5.5-13 µg/L

3-5 µg/L

Total No. Sources

Peak Finding(µg/L)

El Dorado

1

2

3

11

Fresno

1

1

9.2

Kern

2

2

1

5

49

Lake

1

1

2

21

Lassen

1

1

7.0

Los Angeles

2

7

2

11

57

Madera

1

1

1

3

33

Mendocino

1

1

4.9

Merced

1

1

2

6.4

Mono

1

1

7.3

Monterey

4

2

6

400

Riverside

1

3

4

24

Sacramento

2

2

28

San Benito

1

1

36

San Bernardino

1

1

11

San Diego

5

3

1

9

112

San Francisco

1

1

12

San Luis Obispo

1

1

2

13

San Mateo

2

1

3

35

Santa Barbara

1

1

8.6

Santa Cruz

1

1

37

Shasta

1

1

2

44

Siskiyou

1

1

17

Solano

1

1

3.5

Tulare

1

1

2

13

Tuolumne

1

1

11

Yuba

1

1

3.9

TOTAL

24

31

14

69

-

*As of October 1, 2009. Values should be considered draft, since they may change with subsequent updates. Data are from sources with two or more reported MTBE detections (Excel) since January 1, 2000, provided that one detection is at or greater than the DLR.